Carol D. Frost

Reduced rapakivi-type granites: The tholeiite connection

Reduced rapakivi-type granites are the most iron enriched and reduced (i.e., least oxidized) of the “anorogenic” granite association. The low oxygen fugacity and chemical composition of these granites severely limit their sources. In this paper we argue that reduced rapakivi-type granites and their eruptive equivalents, high-potassium fayalite rhyolites, are derived from mafic sources, because tholeiitic magmas and their derivatives have the required low oxygen fugacity. Reduced, rapakivi-type granites are produced either by extreme differentiation of basaltic melts or by partial melting of underplated basalts and their differentiated equivalents. They form in extensional environments where the asthenosphere is present at shallow depths. We envision three stages in the origin of these rocks: (1) tholeiitic melts are emplaced at the base of the crust, (2) continued introduction of heat partially remelts these tholeiitic rocks, and (3) the hot, dry melts so produced migrate into the middle crust to produce rapakivi batholiths or erupt as rhyolites. Partial melting of felsic continental crust may accompany the intrusion of rapakivi-type magmas, thereby producing the other metaluminous and peraluminous granite compositions of the anorogenic suite.

Chemical weathering in the foreland of a retreating glacier

Chemical denudation rates and strontium isotope ratios in streams vary substantially and systematically in the foreland of the retreating Bench Glacier in south-central Alaska. To study weathering of young glacier sediments, we sampled 12 streams draining a chronosequence of till and moraine soils derived from Cretaceous metagraywacke–metapelite bedrock. Both sediment age and vegetation cover increase with distance from the glacier. Cation denudation rates decline with increasing distance from the glacier, whereas silica denudation rates increase. Carbonate dissolution and sulfide oxidation account for roughly 90% of the solute flux from the youngest sediments. Biotite alteration accounts for 5–11% of the solute flux; its peak contribution is found in the glacier outlet stream. Silicate weathering is the dominant reaction only in the oldest sediments. In a laboratory dissolution experiment using fresh glacial sediment, carbonate dissolution dominated the solute flux during the first 700 hours, paralleling the behavior of young sediments in the field. In contrast to trends in the field, the silica flux did not increase after the carbonate was exhausted from the reactor. A possible reason for this difference is that establishment of vegetation causes an increase in silicate weathering. The 87Sr/86Sr ratio in the glacier outlet stream is greater than that in proglacial streams and in bulk rock, due to a greater contribution of biotite weathering in the outlet than in proglacial streams. Strontium isotope ratios decline with sediment exposure age in the proglacial streams, and are consistent with a carbonate source. Because the dominant weathering reactions in the young sediments are of carbonate and sulfide rather than silicate minerals, weathering at the glacier margin is not an important long-term sink for atmospheric CO2.

The Late Archean history of the Wyoming province as recorded by granitic magmatism in the Wind River Range, Wyoming

Abstract The Wyoming province, a small, ca. 500 000 km 2 Archean craton, is the most southwestern of the Archean provinces in North America. It is composed primarily of Late Archean potassium-rich granitic rocks. In contrast to many other Archean provinces, rocks of tonalite-trondhjemite affinity are rare over most of the province and are restricted to rocks older than 2.8 Ga. Field, petrologic, geochemical and isotopic study of the Late Archean granites exposed in the Wind River Range have allowed us to identify at least four periods of potassic calc-alkalic magmatism at ∼2.8, 2.67, 2.63 and 2.55 Ga. Granitic rocks of these ages appear to be widespread across the Wyoming province. The oldest calc-alkalic granites of the Wind River Range, emplaced at ca. 2.8 Ga, appear to be derived predominantly from pre-existing crust. However, Nd isotopic data suggest that these granites cannot be the product solely of partial melting of older tonalitic gray gneisses. During at least two other periods of plutonism, at 2.67 and 2.63 Ga, generation of the Wind River Range batholiths involved the incorporation of substantial amounts of isotopically juvenile material, either from depleted mantle or young continental crust. The information presented below, as well as data available from elsewhere in the Wyoming province, is interpreted to suggest that the Wyoming province, unlike other Archean cratons, is not composed of a tectonic amalgamation of smaller, exotic terranes. Although the Wyoming province did experience crustal addition in Archean time, it was not by lateral accretion, but by incorporation of mantle-derived melts into large granitic batholiths.

Evidence for extensive Proterozoic remobilization of the Aldan Shield and implications for Proterozoic plate tectonic reconstructions of Siberia and Laurentia

Abstract A geological traverse across the Aldan shield along the Aldan River shows that the area is underlain by two distinct rock associations. The Middle Aldan association consists of metasedimentary rocks, mainly quartzite, that have been intruded by a potassic biotite granite. Downstream, north of the Middle Aldan association, lies the Lower Aldan association, which consists mostly of charnockite with rafts of older granulite gneiss that contains abundant metasedimentary layers. UPb dating of zircons from the Middle Aldan association granite (1900 Ma) and Lower Aldan association charnockite (maximum age 1918 Ma) and granitic gneiss (maximum age 2230 Ma) shows that the majority of the rocks exposed along the Aldan River are Proterozoic in age. Although they have Proterozic crystallization ages, the granite, granite gneiss and charnockite yield Archean Nd model ages, suggesting that they formed by remobilization—that is, partial or complete remelting—of earlier Archean crust. In contrast, pelitic gneisses included within the charnockites of the Lower Aldan association give Proterozoic Nd model ages, indicating that a substantial amount of Proterozoic rock is incorporated within the Lower Aldan association. The present results show that the rocks along the Aldan River, which are part of the Aldan block, display a significantly different history from those in the Olekma block to the west. The Olekma block contains ca 3.0 Ga greenstone belts Late Archean amphibolite-grade granitic gneisses; the Proterozoic remobilization that typifies the Aldan terrane is absent. The Olekma block was thrust under the Aldan block during the ca 1.9 Ga orogeny contemporaneous with, or slightly after the 1.9 Ga charnockitic event in the Aldan. The 1.9 Ga magmatic and granulite event seen in the Aldan is similar in age and character to the Thelon magmatic zone of northern Canada. This correlation allows the development of a preferred construction for the Precambrian Laurentia-Siberia connection in which the present day southern portion of the Siberia platform was connected to the northern margin of Laurentia.

Crustal growth by magmatic underplating: Isotopic evidence from the northern Sherman batholith

It is accepted that continental growth takes place both in intraplate and convergent margin settings, but the relative importance of crustal growth in these two tectonic environments is the subject of ongoing debate. In this study we suggest that magmatic underplating in continental interiors results in significant increases in continental volume. We maintain that A-type, or anorogenic, granites are derived from young, underplated mafic crust. Magma sources for A-type granites typically are difficult to identify due to the lack of isotopic contrast between mantle and crustal sources. However, the northern portion of the Mesoproterozoic Sherman batholith, southeastern Wyoming, intrudes Archean gneiss. Isotopic data from these granites preclude derivation from felsic crust, and instead require the involvement of a mantle or mantle-like isotopic reservoir. The data are analogous to those for eruptive equivalents of A-type granites, the fayalite rhyolites of Yellowstone, which also ascended through Archean felsic crust but carry little Archean isotopic signature. Anorogenic granites thus may represent a middle to upper crustal record of magmatic underplating at depth.

High-Al gabbros in the Laramie Anorthosite Complex, Wyoming: implications for the composition of melts parental to Proterozoic anorthosite

High-Al gabbro represents one of the latest phases of magmatism in the 1.43 Ga Laramie anorthosite complex (LAC) in southeastern Wyoming. This lithology, which is mineralogically and geochemically the most primitive in the LAC, forms dikes and small intrusions that cross cut monzonitic and anorthositic rocks. High-Al gabbro is characterized by high Al2O3 (15–19 wt%), REE patterns with positive europium anomalies (Eu/Eu*=1.2–3.8), and the lowest initial 87Sr/86Sr (as low as 0.7033) and highest initial ɛNd (up to +2) in the LAC. Their Sr and Nd isotopic characteristics indicate a mantle origin followed by crustal assimilation during ascent. Intermediate plagioclase (An50–60) and mafic silicate (Fo54–63) compositions suggest that they are not primary mantle melts and that they differentiated prior to final emplacement. High-Al gabbros of the LAC are similar compositionally to gabbros from several other Proterozoic anorthosite complexes, including rocks from the Harp Lake complex and the Hettasch intrusion in Labrador and the Adirondack Mountains of New York. These gabbros are considered to be parental to their associated anorthositic rocks, a theory that is supported by recent experimental work. We interpret LAC high-Al gabbros to represent mantle-derived melts produced by the differentiation of a basaltic magma in an upper mantle chamber. Continued evolution of this magma eventually resulted in the formation of plagioclase-rich diapirs which ascended to mid-crustal levels and formed the anorthositic rocks of the LAC. Because these gabbros intrude the anorthositic rocks, they do not represent directly the magma from which anorthosite crystallized and instead are younger samples of magma formed by identical processes.

The relationship between A-type granites and residual magmas from anorthosite: evidence from the northern Sherman batholith, Laramie Mountains, Wyoming, USA

Abstract Anorthosite complexes commonly are spatially and temporally associated with A-type granite batholiths, but their genetic relationship, if any, has been difficult to determine. This study focuses on the northern portion of the 1.43–1.44 Ga Sherman batholith, which is located adjacent to the 1.44 Ga Laramie anorthosite complex, Laramie Mountains, Wyoming, USA. The northern Sherman batholith is an ideal place to resolve the relationship of A-type granite with anorthosite because of the age of the crust into which it is emplaced. Most A-type granites are intruded into crust only slightly older than the granites themselves, and there is little isotopic contrast between potential mantle and crustal source rocks. The northern Sherman batholith intrudes Archean crust that by 1.44 Ga had developed Nd, Sr and Pb isotopic compositions that differed greatly from contemporaneous mantle. The northern Sherman batholith is an iron-enriched, metaluminous, alkalic to alkali-calcic batholith. Most of the batholith is composed of fayalite granite, with minor volumes of monzodiorite and olivine gabbro. Hypabyssal dikes of these compositions crop out in the highest structural levels exposed in the area. The northern Sherman batholith is less differentiated and has assimilated less felsic crust than the southern portions, which intrude more fertile Proterozoic crust. Northern Sherman batholith rocks are compositionally and isotopically very similar to monzonitic and dioritic rocks of the Laramie anorthosite complex. None of these rocks have isotopic compositions similar to their Archean host rocks, precluding derivation solely from such sources. Instead, northern Sherman batholith granites, like Laramie anorthosite complex rocks, are related to mantle-derived tholeiitic magmas emplaced at the base of the crust. The tectonic environment and magmatic processes that lead to the formation of massif anorthosite are also conducive to the formation of voluminous A-type granite, of which the northern Sherman granite is a prime example.

Mid-Pleistocene lavas from the Seguam volcanic center, central Aleutian arc: closed-system fractional crystallization of a basalt to rhyodacite eruptive suite

In contrast to adjacent volcanic centers of the modern central Aleutian arc, Seguam Island developed on strongly extended arc crust. K-Ar dates indicate that mid-Pleistocene, late-Pleistocene, and Holocene eruptive phases constitute Seguam. This study focuses on the petrology of the mid-Pleistocene, 1.07–07 Ma, Turf Point Formation (TPF) which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst-poor andesite to rhyodacite lavas. Increasing whole-rock FeO*/MgO from basalt to dacite, the anhydrous Plag+Ol+Cpx±Opx±Mt phenocryst assemblage, groundmass pigeonite, and the reaction Ol+Liq=Opx preserved in the mafic lavas indicate a tholeiitic affinity. Thermometry and comparison to published phase equilibria suggests that most TPF basalts crystallized Plag+Ol+Cpx±Mt at ≥1160°C between about 3–5 kb (±1–2% H2O), andesites crystallized Plag+Cpx+Opx±Mt at ≥1000°C between 3–4 kb with 3–5% H2O, and dacites crystallized Plag +Cpx±Opx±Mt at 1000°C between 1–2 kb with 2–3% H2O. All lavas crystallized at fo2 close to the NNO buffer. Mineral compositions and textures indicate equilibrium crystallization of the evolved lavas; petrographic evidence of open-system mixing or assimilation is rare. MgO, CaO, Al2O3, Cr, Ni, and Sr abundances decrease and K2O, Na2O, Rb, Ba, Zr, and Pb increase with increasing SiO2 (50–71%). LREE enrichment [(Ce/Yb)n=1.7±0.2] characterizes most TPF lavas; total REE contents increase and Eu anomalies become more negative with increasing SiO2. Relative to other Aleutian volcanic centers, TPF basalts and basaltic andesites have lower K2O, Na2O, TiO2, Rb, Ba, Sr, Zr, Y, and LREE abundances. 87Sr/86Sr ratios (0.70361–0.70375) and ratios of 206Pb/204Pb (18.88–18.97), 207Pb/204Pb (15.58–15.62), 208Pb/204Pb (38.46–38.55) are the highest measured for any suite of lavas in the oceanic portion of the Aleutian arc. Conversely, εNd values (+5.8 to+6.7) are among the lowest from the Aleutians. Sr, Nd, and Pb ratios are virtually constant from basalt through rhyodacite, whereas detectable isotopic heterogenity is observed at most other Aleutian volcanic centers. Major and trace element, REE, and Sr, Nd, and Pb isotopic compositions are consistent with the basaltic andesitic, andesitic, dacitic, and rhyodacitic liquids evolving from TPF basaltic magma via closed-system fractional crystallization alone. Fractionation models suggest that removal of ∼80 wt% cumulate (61% Plag, 17% Cpx, 12% Opx, 7% Ol, and 3% Mt) can produce 20 wt% rhyodacitic residual liquid per unit mass of parental basaltic liquid. Petrologic and physical constraints favor segregation of small batches of basalt from a larger mid-crustal reservoir trapped below a low-density upper crustal lid. In these small magma batches, the degree of cooling, crystallization, and fractionation are functions of the initial mass of basaltic magma segregated, the thermal state of the upper crust, and the magnitude of extension. Tholeiitic magmas erupted at Seguam evolved by substantially different mechanisms than did calc-alkaline lavas erupted at the adjacent volcanic centers of Kanaga and Adak on unextended arc crust. These variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew. At Seguam, intra-arc extension promoted eruption of voluminous basalt and its differentiates, unmodified by interaction with lower crustal or upper mantle wallrocks.

Residual-liquid origin for a monzonitic intrusion in a mid-Proterozoic anorthosite complex: The Sybille intrusion, Laramie anorthosite complex, Wyoming

The Sybille intrusion (≈100 km 2) is one of three large monzonitic intrusions in the 1.43 Ga Laramie anorthosite complex of southeastern Wyoming. The petrographic, geochemical, isotopic, and geophysical characteristics of Sybille monzonitic rocks are consistent with an origin by extensive crystallization of liquids residual to nearby anorthositic cumulates (ferrodiorites) and contamination by Archean wall rocks. The exposed part of the intrusion is composed mainly of coarse-grained monzosyenites with abundant alkali feldspar phenocrysts. The monzosyenites preserve mineralogical evidence for high crystallization temperatures (>1000 °C), mid-crustal emplacement pressures (≈3 kbar), relatively reduced crystallization conditions (2 log units below the fayalite + magnetite + quartz [FMQ] oxygen buffer), and they crystallized in the presence of a CO2-rich fluid phase (Fuhrman et al., 1988; Frost and Touret, 1989). The eastern monzosyenites, those adjacent to contemporaneous anorthosite, are distinguished by an anhydrous mineral assemblage (Fo16-Fo8 olivine, high-Ca pyroxene) lacking modal quartz, silica contents of 60 wt%, and smaller Eu anomalies (Eu/Eu* = 1.2 to 1.3). Abundant xenoliths of Archean wall rocks and anorthosite from the adjacent intrusions in all monzosyenites attest to a stoping emplacement mechanism near the roof of the chamber. We propose that the monzosyenites represent a relatively thin, 0.5-1.0-km-thick, roof to a magma chamber dominated by dense ferrodioritic cumulates at depth. Extensive, open-system fractionation of a ferrodioritic parent magma, residual after crystallization of anorthosite, produced Fe-enriched monzodioritic and/or monzonitic magma in the upper part of the chamber and complementary Fe- and Ti-rich cumulates in the lower levels. We have corroborated the production of monzonitic liquids from crystallization of ferrodiorite through a series of reconnaissance equilibrium-crystallization experiments. The presence of dense ferrodioritic cumulates at depth is consistent with the prominent positive gravity anomaly associated with the Sybille intrusion (Hodge et al., 1973). In the upper parts of the chamber, the fractionated monzodioritic and/or monzonitic magmas eventually became saturated in alkali feldspar. Owing to density contrasts, the alkali feldspar phenocrysts floated to the roof of the chamber, thus producing the exposed porphyritic monzosyenites. In addition, the roof of the chamber was the site of significant melting of Archean gneiss and, locally, metapelite. The Sr and Nd isotopic compositions of the monzosyenites, with Sr isotopic ratios becoming increasingly radiogenic from east ( I Sr = 0.7059 and initial ϵNd = −2.5) to west ( I Sr = 0.7092 and initial ϵNd = −2.6), are consistent with a 5% to 15% addition of Archean orthogneiss to a ferrodioritic parent magma that had isotopic characteristics similar to adjacent anorthositic rocks. The stratigraphic and compositional similarity of the Sybille monzosyenites to mangerites in the Bjerkreim-Sokndal intrusion of the Rogaland anorthosite complex, southern Norway, indicates that similar open-system magmatic processes are capable of having produced high-temperature, K-rich monzonitic rocks in other Proterozoic anorthosite complexes.

Nd isotopic evidence for the antiquity of the Wyoming province

Sm-Nd isotopic data on Late Archean age metasedimentary rocks from around the Wyoming province yield ϵ Nd values at 2.6 Ga from -12.9 to +3.3, which correspond to depleted mantle model ages of 2.7 to 3.8 Ga. These results indicate that crust of a wide variety of ages was exposed and eroding in Late Archean time. The predominance of Middle Archean model ages suggests that continental crust was extensive in the Wyoming province by this time. No resolvable correlation exists between initial ϵ Nd value and geographic location or depositional environment of the metasedimentary rock samples; thus, there is no indication that any part of the province was established at a time different from the rest. The Nd isotopic characteristics of Archean rocks from the Wyoming province contrast with those of the Superior province, confirming that, although their Proterozoic evolution has been parallel, the two cratons did not share a common Archean history.